For recent shrinkage of semiconductor devices, further reduction in resistance and capacitance is required to contribute to higher speed and lower power consumption. For example, when attention is focused on the capacitance reduction, there is a pressing need to develop an interlayer insulating film with a low dielectric constant. For example, a semiconductor device having an air gap in a space between wiring lines is known as a method of reducing the dielectric constant.
As a technique for forming the air gap, a method is known which uses CVD providing low coverage in forming a coating film. However, the space between the wiring lines is entirely or partly covered even when conditions that provide low step coverage are used.
Thus, it has heretofore been not easy to form an air gap which allows the space between the wiring lines to be hardly filled with an insulating film, that is, which shows high percentage of void.